1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer-readable storage medium for correcting the brightness of image data.
2. Description of the Related Art
Conventionally, many apparatuses and methods have been proposed for printing a preferred photographic image by performing various kinds of correction on photographic image data captured by a digital camera. These image correction methods can be broadly classified into two types, uniform correction in which uniform correction is performed on the whole image, and local correction in which a correction amount varies based on the local properties of the image. A representative example of local correction is exposure dodging correction.
Dodging correction is, for example, when an object, such as a person, is dark and the background is bright, the luminosity of the dark person area is greatly increased, and the brightness of the bright background area is not changed very much. Consequently, background overexposure is suppressed, and the brightness of the person area is appropriately corrected.
Examples of such a dodging process are discussed in Japanese Patent Application Laid-Open No. 2001-298619 and Japanese Patent No. 3233114. Both Japanese Patent Application Laid-Open No. 2001-298619 and Japanese Patent No. 3233114 realize dodging correction of a digital image by performing filter processing on an input image to generate a low-frequency image, i.e., a blurry image, and using this blurry image as a brightness control factor.
Further, an example of a characteristic common to Japanese Patent Application Laid-Open No. 2001-298619 and Japanese Patent No. 3233114 is the feature of compositing a plurality of blurry images (low-frequency images) having different blurriness levels, and performing dodging processing using this composite low-frequency image. By compositing a plurality of blurry images having different blurriness levels, a composite blurry image, which stores a certain amount of border edges between the object and the background, can be generated. If dodging processing is performed on the input image using low-frequency images having one type of blurriness level, a fixed width false contour is produced on the periphery of the object. Therefore, a composite low-frequency image is used for dodging processing.
When performing dodging processing using the composite low-frequency image like that described above, there are the following problems. For example, when printing an image having an input image size of width 3,000×height 4,500 pixels at 300 dpi on a four-inch wide, six-inch high sheet of printing paper, the required output image size is 1,200×1,800 pixels.
In this case, if the dodging processing is performed on the output image using a composite low-frequency image having the same size as the input image, which is generated by performing filtering processing on the whole input image, the generated composite low-frequency image will have 6.25 times the number of pixels than the size of the output image. Thus, wastefulness occurs in the filtering processing and the composite processing. Needless to say, if the printing paper has a smaller size, such as an L size or a business card size, the difference between the composite low-frequency image and the output image sizes will increase still further.
This wastefulness of the processing can be ignored for recent personal computers, which have very high CPU capabilities and memory access speed. However, for hardware that can only hold a very limited calculation capability, such as a low-cost inkjet printer, or a large-scale print system that requires a printing capability of several hundred sheets per minute, this processing wastefulness can be a factor that reduces overall performance.
For this reason, when performing dodging processing on an input image by generating a composite low-frequency image, it is desirable to composite low-frequency images having required resolution by considering the printing application, such as what size of paper the printing will be carried out on. Although generating low-frequency images having the required resolution to allow the calculation amount of the composite low-frequency image to be suppressed is preferable from perspective of a processing speed, such a technique has not been proposed.